1. Field of the Invention
The present invention relates generally to processes for sulfur recovery and more specifically to processes for removing sulfur compounds, including hydrogen sulfide and sulfur dioxide, from refinery and like waste gas streams.
2. Description of the Related Art
Processing of hydrocarbon-containing fuels such as gasoline and diesel fuel results in gases containing sulfur compounds, including hydrogen sulfide (H2S), and hydrocarbon compounds, referred to herein from time to time as waste gas or offgas streams. Governmental regulations limit plant emissions of sulfur-bearing gases. Refineries commonly include sulfur reduction units to decrease emissions of sulfur compounds.
The use of a Claus catalytic reaction is widely known in the field and commonly used in sulfur recovery units. The currently practiced modified Claus process consists of a thermal stage and a catalytic stage. In the thermal stage, a waste gas containing hydrogen sulfide is injected into a thermal reactor where hydrogen sulfide is partially oxidized with air at high temperatures to form a quantity of sulfur dioxide. The thermal reaction further serves to oxidize ammonia. Combustion gases are cooled in a waste heat boiler in which a portion of the hydrogen sulfide reacts with sulfur dioxide to form water and elemental sulfur. The elemental sulfur is condensed and removed.
In the Clause process catalytic stage, remaining gases are transmitted to a series of reactors to further react hydrogen sulfide and sulfur dioxide to remove elemental sulfur. Typically the reactors utilize a catalyst, such as aluminum oxide, titanium oxide or bauxite. Typical Claus process catalytic stages include heating the process gas, reaction in the catalytic reactor, condensation of the elemental sulfur, and further transmission of the remaining process gases. One to four reactor stages are typically practiced; however, a typical process involves two or three reactors. Tail gas treatment processes further treat remaining process gas after the last reactor.
The Claus process involves reaction of sulfur dioxide (SO2) and hydrogen sulfide (H2S) react to produce elemental sulfur (S2) and steam/water (H20). The reaction formula is:2H2S+SO2→1.5S2+2H2O
Stoichiometric balance of two moles of hydrogen sulfide to one mole of sulfur dioxide in the process is historically difficult to achieve due to, among other things, variations in composition of off-gas streams.
The present invention provides a process to obtain stoichiometric balance by introducing determined flow quantities of sulfur dioxide into the process stream at determined locations during a multiple stage reactor process, analyzing hydrogen sulfide content at one or more locations, and adjusting input flows of sulfur dioxide into the process stream at one or more locations. Advantages of the present invention include reduced total mass flow during the process and consequently reduce pressure drop, thereby providing improved process efficiency and increased capacity of existing Claus process facilities. In an embodiment of the present invention wherein sulfur dioxide is introduced into the thermal reactor, the injected sulfur dioxide reduces combustion air required in the thermal reactor, thereby reducing mass flow of, among other things, water and nitrogen, and correspondingly reduces pressure drop and increases efficiency of a given sulfur recovery unit.